Police use of force has been heavily scrutinized for more than a year after fatal police encounters with unarmed black men in New York City, Ferguson, Mo., Baltimore and other parts of the U.S. The increased attention has renewed calls for law-enforcement officers to wear video cameras while on duty.

Supporters say the devices are needed to provide transparency, build public trust and provide evidence against false complaints. But as more law-enforcement agencies begin using them, questions emerge as to when they should be turned on and off and how much footage should be made available to the public.